Submarine optical cable systems are required to transmit optical signals over a distance longer than thousands of kilometers, and therefore the systems typically use optically amplifying repeaters in predetermined repeating intervals to optically amplify attenuated optical signals.
Optical fibers currently used for transmission have diameters of their cores 8 to 8.5 .mu.m (with effective cross-sectional areas of approximately 50 to 55 .mu.m.sup.2), and used as optically amplifying repeaters are erbium-doped optical fibers pumped with 1.48 .mu.m. By combining these elements, transmission of 5 Gb/s over 9,000 km, like optical submarine cable TPC-5 connecting Japan and U.S.A., is currently used in practice.
Transmission capacity can be increased by the number of wavelengths by simultaneously using wavelength-division multiplexing technologies. For example, although depending on the distance of transmission, it has been confirmed that NRZ systems can multiplex five to twenty wavelengths and optical soliton systems can multiplex five to eight wavelengths.
Erbium-doped optical fibers have absorption levels of 1.48 Jim band and 0.9 .mu.m band. Semiconductor lasers for 0.98 .mu.m, however, have been unreliable for use in transmission. Moreover, when such optical fibers are pumped with 0.98 .mu.m, optimum amplifier outputs cannot be set high, taking nonlinear effects into account, as compared with pumping with 1.48 .mu.m. Therefore, the band of 1.48 .mu.m has been employed in practice.
However, signals must be repeated and transmitted, for example, over 9,000 km for directly connecting Japan and U.S.A., and 11,000 km for directly connecting U.S.A. and China. For realizing transmission capacity of 100 Gb/s as a whole by multiplexing 20 wavelengths of optical signals of 5 Gb/s, it is difficult to achieve the optically amplifying/repeating transmission over 9,000 km or more by the combinations of existing optical fiber cables and optical amplifiers.
Assuming that optical transmission fibers have the same transmission loss, it is necessary to increase power of optical signal outputs of optically amplifying repeaters or to greatly decrease the amount of noise of optical amplifiers, in order to increase the transmission distance. However, an increase in power of optical signals accompanies deterioration in transmission characteristics due to nonlinear effects of optical transmission fibers, and expected bands cannot be ensured. It is also difficult to decrease noise with erbium-doped optical fibers pumped by laser light of 1.48 .mu.m.